Conventional system on a chip (SOC) designs need a real time clock. For example, a DVD recorder (or other recorder) needs to know when to wake up from a sleep mode to record a particular event, such as a TV program. The clock needs to keep the time even when the recorder is unplugged from the power supply. In particular, a quality recorder should not be blinking at “12:00” when the power supply is restored.
Conventional solutions use an external discrete real time clock (RTC) chip to hold time. Such chips normally use bipolar technology so that they are able to run on a button battery or other low voltage inexpensive battery. External discrete real time clock chips increase the overall cost of a design.
Other solutions integrate the clock onto the SOC chip. However, a SOC is not ideal for extremely low power applications (i.e., being powered by a button battery). A typical integrated clock needs to run on a re-chargeable battery only. Rechargeable batteries are expensive. Another approach is to integrate a clock into the SOC chip with special process. Such special processes are expensive and not practical on sub-micron technology.
It would be desirable to implement an inexpensive real time clock that minimized power usage.